Streamlined Linking with OAuth and Google Sign-In

OAuth-based Google Sign-In Streamlined linking adds Google Sign-In on top of OAuth linking. This provides a seamless linking experience for Google users, and it also enables account linking for users who registered to your service with a non-Google identity.

To perform account linking with OAuth and Google Sign-In, follow these general steps:

First, ask the user to give consent to access their Google profile.
Use the information in their profile to check if the user account exists.
For existing users, link the accounts.
If you can't find a match for the Google user in your authentication system, validate the ID token received from Google. You can then create a user based on the profile information contained in the ID token.

Accounts are linked using industry standard OAuth 2.0 implicit and authorization code flows. Your service must support OAuth 2.0-compliant authorization and token exchange endpoints. Additionally, your token exchange endpoint should support JSON Web Token (JWT) assertions and implement the check, create, and get intents.

In the implicit flow, Google opens your authorization endpoint in the user's browser. After successful sign in, you return a long-lived access token to Google. This access token is now included in every request sent from Google.

In the authorization code flow, you need two endpoints:

The authorization endpoint, which presents the sign-in UI to your users that aren't already signed in. The authorization endpoint also creates a short-lived authorization code to record users' consent to the requested access.
The token exchange endpoint, which is responsible for two types of exchanges:
1. Exchanges an authorization code for a long-lived refresh token and a short-lived access token. This exchange happens when the user goes through the account linking flow.
2. Exchanges a long-lived refresh token for a short-lived access token. This exchange happens when Google needs a new access token because the one it had expired.

Choose an OAuth 2.0 flow

Although the implicit flow is simpler to implement, Google recommends that access tokens issued by the implicit flow never expire. This is because the user is forced to link their account again after a token expires with the implicit flow. If you need token expiration for security reasons, we strongly recommend that you use the authorization code flow instead.

Design guidelines

This section describes the design requirements and recommendations for the user screen that you host for OAuth linking flows. After it's called by Google's app, your platform displays a sign in to Google page and account linking consent screen to the user. The user is directed back to Google's app after giving their consent to link accounts.

This figure shows the steps for a user to link their Google account
to your authentication system. The first screenshot shows
user-initiated linking from your platform. The second image shows
user sign-in to Google, while the third shows the user consent and
confirmation for linking their Google account with your app. The
final screenshot shows a successfully linked user account in the
Google app. — **Figure 1.** Account linking user sign in to Google and consent screens.

Requirements

You must communicate that the user’s account will be linked to Google, not a specific Google product like Google Home or Google Assistant.

Recommendations

We recommend that you do the following:

Display Google's Privacy Policy. Include a link to Google’s Privacy Policy on the consent screen.
Data to be shared. Use clear and concise language to tell the user what data of theirs Google requires and why.
Clear call-to-action. State a clear call-to-action on your consent screen, such as “Agree and link.” This is because users need to understand what data they're required to share with Google to link their accounts.
Ability to cancel. Provide a way for users to go back or cancel, if they choose not to link.
Clear sign-in process. Ensure that users have clear method for signing in to their Google account, such as fields for their username and password or Sign in with Google.
Ability to unlink. Offer a mechanism for users to unlink, such as a URL to their account settings on your platform. Alternatively, you can include a link to Google Account where users can manage their linked account.
Ability to change user account. Suggest a method for users to switch their account(s). This is especially beneficial if users tend to have multiple accounts.
- If a user must close the consent screen to switch accounts, send a recoverable error to Google so the user can sign in to the desired account with OAuth linking and the implicit flow.
Include your logo. Display your company logo on the consent screen. Use your style guidelines to place your logo. If you wish to also display Google's logo, see Logos and trademarks.

Configure the project

To configure your project to use OAuth account linking:

Go to the Google API Console.
Click Create project.
Enter a name or accept the generated suggestion.
Confirm or edit any remaining fields.
Click Create.

To view your project ID:

Go to the Google API Console.
Find your project in the table on the landing page. The project ID appears in the ID column.

Implement your OAuth server

To support the OAuth 2.0 implicit flow, your service makes an authorization endpoint available by HTTPS. This endpoint is responsible for authentication and obtaining consent from users for data access. The authorization endpoint presents a sign-in UI to your users that aren't already signed in and records consent to the requested access.

When a Google application needs to call one of your service's authorized APIs, Google uses this endpoint to get permission from your users to call these APIs on their behalf.

A typical OAuth 2.0 implicit flow session initiated by Google has the following flow:

Google opens your authorization endpoint in the user's browser. The user signs in, if not signed in already, and grants Google permission to access their data with your API, if they haven't already granted permission.
Your service creates an access token and returns it to Google. To do so, redirect the user's browser back to Google with the access token attached to the request.
Google calls your service's APIs and attaches the access token with each request. Your service verifies that the access token grants Google authorization to access the API and then completes the API call.

Handle authorization requests

When a Google application needs to perform account linking via an OAuth 2.0 implicit flow, Google sends the user to your authorization endpoint with a request that includes the following parameters:

Authorization endpoint parameters
`client_id`	The client ID you assigned to Google.
`redirect_uri`	The URL to which you send the response to this request.
`state`	A bookkeeping value that is passed back to Google unchanged in the redirect URI.
`response_type`	The type of value to return in the response. For the OAuth 2.0 implicit flow, the response type is always `token`.
`user_locale`	The Google Account language setting in RFC5646 format used to localize your content in the user's preferred language.

For example, if your authorization endpoint is available at https://myservice.example.com/auth, a request might look like the following:

GET https://myservice.example.com/auth?client_id=GOOGLE_CLIENT_ID&redirect_uri=REDIRECT_URI&state=STATE_STRING&response_type=token&user_locale=LOCALE

For your authorization endpoint to handle sign-in requests, do the following steps:

Verify the client_id and redirect_uri values to prevent granting access to unintended or misconfigured client apps:
- Confirm that the client_id matches the client ID you assigned to Google.
- Confirm that the URL specified by the redirect_uri parameter has the following form:
```
https://oauth-redirect.googleusercontent.com/r/YOUR_PROJECT_ID
https://oauth-redirect-sandbox.googleusercontent.com/r/YOUR_PROJECT_ID
```
Check if the user is signed in to your service. If the user isn't signed in, complete your service's sign-in or sign-up flow.
Generate an access token for Google to use to access your API. The access token can be any string value, but it must uniquely represent the user and the client the token is for and must not be guessable.
Send an HTTP response that redirects the user's browser to the URL specified by the redirect_uri parameter. Include all of the following parameters in the URL fragment:
- access_token: The access token you just generated
- token_type: The string bearer
- state: The unmodified state value from the original request
The following is an example of the resulting URL:
```
https://oauth-redirect.googleusercontent.com/r/YOUR_PROJECT_ID#access_token=ACCESS_TOKEN&token_type=bearer&state=STATE_STRING
```

Google's OAuth 2.0 redirect handler receives the access token and confirms that the state value hasn't changed. After Google has obtained an access token for your service, Google attaches the token to subsequent calls to your service APIs.

Check for an existing user account

After the user gives consent to access their Google profile, Google sends a request that contains a signed assertion of the Google user's identity. The assertion contains information that includes the user's Google Account ID, name, and email address. The token exchange endpoint configured for your project handles that request.

If the corresponding Google account is already present in your authentication system, your token exchange endpoint responds with account_found=true. If the Google account doesn't match an existing user, your token exchange endpoint returns an HTTP 404 Not Found error with account_found=false.

The request has the following form:

POST /token HTTP/1.1
Host: oauth2.example.com
Content-Type: application/x-www-form-urlencoded

grant_type=urn:ietf:params:oauth:grant-type:jwt-bearer&intent=check&assertion=JWT&scope=SCOPES

Your token exchange endpoint must be able to handle the following parameters:

Token endpoint parameters
`intent`	For these requests, the value of this parameter is `check`.
`grant_type`	The type of token being exchanged. For these requests, this parameter has the value `urn:ietf:params:oauth:grant-type:jwt-bearer`.
`assertion`	A JSON Web Token (JWT) that provides a signed assertion of the Google user's identity. The JWT contains information that includes the user's Google Account ID, name, and email address.

When your token exchange endpoint receives the check request, it needs to validate and decode the JWT assertion.

Validate and decode the JWT assertion

You can validate and decode the JWT assertion by using a JWT-decoding library for your language. Use Google's public keys, available in JWK or PEM formats, to verify the token's signature.

When decoded, the JWT assertion looks like the following example:

{
  "sub": "1234567890",      // The unique ID of the user's Google Account
  "iss": "https://accounts.google.com",        // The assertion's issuer
  "aud": "123-abc.apps.googleusercontent.com", // Your server's client ID
  "iat": 233366400,         // Unix timestamp of the assertion's creation time
  "exp": 233370000,         // Unix timestamp of the assertion's expiration time
  "name": "Jan Jansen",
  "given_name": "Jan",
  "family_name": "Jansen",
  "email": "jan@gmail.com", // If present, the user's email address
  "email_verified": true,   // true, if Google has verified the email address
  "hd": "example.com",      // If present, the host domain of the user's GSuite email address
                            // If present, a URL to user's profile picture
  "picture": "https://lh3.googleusercontent.com/a-/AOh14GjlTnZKHAeb94A-FmEbwZv7uJD986VOF1mJGb2YYQ",
  "locale": "en_US"         // User's locale, from browser or phone settings
}

In addition to verifying the token's signature, verify that the assertion's issuer (iss field) is https://accounts.google.com, that the audience (aud field) is your assigned client ID, and that the token has not expired (exp field).

Using the email, email_verified and hd fields you can determine if Google hosts and is authoritative for an email address. In cases where Google is authoritative the user is currently known to be the legitimate account owner and you may skip password or other challenges methods. Otherwise, these methods can be used to verify the account prior to linking.

Cases where Google is authoritative:

email has a @gmail.com suffix, this is a Gmail account.
email_verified is true and hd is set, this is a G Suite account.

Users may register for Google Accounts without using Gmail or G Suite. When email does not contain a @gmail.com suffix and hd is absent Google is not authoritative and password or other challenge methods are recommended to verify the user. email_verfied can also be true as Google initially verified the user when the Google account was created, however ownership of the third party email account may have since changed.

Check if the Google account is already present in your authentication system

Check whether either of the following conditions are true:

The Google Account ID, found in the assertion's sub field, is in your user database.
The email address in the assertion matches a user in your user database.

If either condition is true, the user has already signed up. In that case, return a response like the following:

HTTP/1.1 200 Success
Content-Type: application/json;charset=UTF-8

{
  "account_found":"true",
}

Google then displays a linking consent dialog to the user and requests consent for the desired scopes in order to continue with linking. After Google obtains the user's consent, Google sends a get request to your token endpoint to continue the linking.

If neither the Google Account ID nor the email address specified in the assertion matches a user in your database, the user hasn't signed up yet. In this case, your token exchange endpoint needs to reply with a HTTP 404 error that specifies "account_found": "false", as in the following example:

HTTP/1.1 404 Not found
Content-Type: application/json;charset=UTF-8

{
  "account_found":"false",
}

When Google receives the 404 error response with a "account_found": "false" error, Google displays a dialog to the user to request consent to create a new account and access to the desired scopes. After Google obtains the user's consent, Google calls your token exchange endpoint with the value of the intent parameter set to create and includes an ID token that contains the user's profile information with the request.

Handle automatic linking

If the corresponding Google Account is already present in your authentication system, your token exchange endpoint returns a token for the user. If the Google Account doesn't match an existing user, your token exchange endpoint returns a linking_error error and optional login_hint.

The request has the following form:

POST /token HTTP/1.1
Host: oauth2.example.com
Content-Type: application/x-www-form-urlencoded

grant_type=urn:ietf:params:oauth:grant-type:jwt-bearer&intent=get&assertion=JWT&scope=SCOPES

Your token exchange endpoint must be able to handle the following parameters:

Token endpoint parameters
`intent`	For these requests, the value of this parameter is `get`.
`grant_type`	The type of token being exchanged. For these requests, this parameter has the value `urn:ietf:params:oauth:grant-type:jwt-bearer`.
`assertion`	A JSON Web Token (JWT) that provides a signed assertion of the Google user's identity. The JWT contains information that includes the user's Google Account ID, name, and email address.
`scope`	Optional: Any scopes that you've configured Google to request from users.

When your token exchange endpoint receives the linking request, it needs to validate and decode the JWT assertion.

Validate and decode the JWT assertion

You can validate and decode the JWT assertion by using a JWT-decoding library for your language. Use Google's public keys, available in JWK or PEM formats, to verify the token's signature.

When decoded, the JWT assertion looks like the following example:

{
  "sub": "1234567890",      // The unique ID of the user's Google Account
  "iss": "https://accounts.google.com",        // The assertion's issuer
  "aud": "123-abc.apps.googleusercontent.com", // Your server's client ID
  "iat": 233366400,         // Unix timestamp of the assertion's creation time
  "exp": 233370000,         // Unix timestamp of the assertion's expiration time
  "name": "Jan Jansen",
  "given_name": "Jan",
  "family_name": "Jansen",
  "email": "jan@gmail.com", // If present, the user's email address
  "email_verified": true,   // true, if Google has verified the email address
  "hd": "example.com",      // If present, the host domain of the user's GSuite email address
                            // If present, a URL to user's profile picture
  "picture": "https://lh3.googleusercontent.com/a-/AOh14GjlTnZKHAeb94A-FmEbwZv7uJD986VOF1mJGb2YYQ",
  "locale": "en_US"         // User's locale, from browser or phone settings
}

Cases where Google is authoritative:

email has a @gmail.com suffix, this is a Gmail account.
email_verified is true and hd is set, this is a G Suite account.

Check if the Google account is already present in your authentication system

Check whether either of the following conditions are true:

The Google Account ID, found in the assertion's sub field, is in your user database.
The email address in the assertion matches a user in your user database.

In some cases, account linking based on ID token might fail for the user. If it does so for any reason, your token exchange endpoint needs to reply with a HTTP 401 error that specifies error=linking_error, as the following example shows:

HTTP/1.1 401 Unauthorized
Content-Type: application/json;charset=UTF-8

{
  "error":"linking_error",
  "login_hint":"foo@bar.com"
}

When Google receives a 401 error response with linking_error, Google calls your token exchange endpoint with the following in the request:

The intent parameter set to create
A JWT with the ID token and user's profile information

Handle account creation via Google Sign-In

When a user needs to create an account on your service, Google makes a request to your token exchange endpoint that specifies intent=create.

The request has the following form:

POST /token HTTP/1.1
Host: oauth2.example.com
Content-Type: application/x-www-form-urlencoded

response_type=token&grant_type=urn:ietf:params:oauth:grant-type:jwt-bearer&scope=SCOPES&intent=create&assertion=JWT[&NEW_ACCOUNT_INFO]

Your token exchange endpoint must able to handle the following parameters:

Token endpoint parameters
`intent`	For these requests, the value of this parameter is `create`.
`grant_type`	The type of token being exchanged. For these requests, this parameter has the value `urn:ietf:params:oauth:grant-type:jwt-bearer`.
`assertion`	A JSON Web Token (JWT) that provides a signed assertion of the Google user's identity. The JWT contains information that includes the user's Google Account ID, name, and email address.

The JWT within the assertion parameter contains the user's Google Account ID, name, and email address, which you can use to create a new account on your service.

To respond to account creation requests, your token exchange endpoint must perform the steps in the following two sections.

Validate and decode the JWT assertion

You can validate and decode the JWT assertion by using a JWT-decoding library for your language. Use Google's public keys, available in JWK or PEM formats, to verify the token's signature.

When decoded, the JWT assertion looks like the following example:

{
  "sub": "1234567890",      // The unique ID of the user's Google Account
  "iss": "https://accounts.google.com",        // The assertion's issuer
  "aud": "123-abc.apps.googleusercontent.com", // Your server's client ID
  "iat": 233366400,         // Unix timestamp of the assertion's creation time
  "exp": 233370000,         // Unix timestamp of the assertion's expiration time
  "name": "Jan Jansen",
  "given_name": "Jan",
  "family_name": "Jansen",
  "email": "jan@gmail.com", // If present, the user's email address
  "email_verified": true,   // true, if Google has verified the email address
  "hd": "example.com",      // If present, the host domain of the user's GSuite email address
                            // If present, a URL to user's profile picture
  "picture": "https://lh3.googleusercontent.com/a-/AOh14GjlTnZKHAeb94A-FmEbwZv7uJD986VOF1mJGb2YYQ",
  "locale": "en_US"         // User's locale, from browser or phone settings
}

Cases where Google is authoritative:

email has a @gmail.com suffix, this is a Gmail account.
email_verified is true and hd is set, this is a G Suite account.

Validate user information and create new account

Check whether either of the following conditions are true:

The Google Account ID, found in the assertion's sub field, is in your user database.
The email address in the assertion matches a user in your user database.

If either condition is true, prompt the user to link their existing account with their Google Account. To do so, respond to the request with an HTTP 401 error that specifies error=linking_error and gives the user's email address as the login_hint. The following is a sample response:

HTTP/1.1 401 Unauthorized
Content-Type: application/json;charset=UTF-8

{
  "error":"linking_error",
  "login_hint":"foo@bar.com"
}

When Google receives a 401 error response with linking_error, Google sends the user to your authorization endpoint with login_hint as a parameter. The user completes account linking using the OAuth linking flow in their browser.

If neither condition is true, create a new user account with the information provided in the JWT. New accounts don't typically have a password set. It's recommended that you add Google Sign-In to other platforms to enable users to log in with Google across the surfaces of your application. Alternatively, you can email the user a link that starts your password recovery flow to allow the user to set a password to sign in on other platforms.

When the creation is completed, issue an access token and return the values in a JSON object in the body of your HTTPS response, like in the following example:

{
  "token_type": "Bearer",
  "access_token": "ACCESS_TOKEN",

  "expires_in": SECONDS_TO_EXPIRATION
}

Validating your implementation

You can validate your implementation by using the OAuth 2.0 Playground tool.

In the tool, do the following steps:

Click Configuration to open the OAuth 2.0 Configuration window.
In the OAuth flow field, select Client-side.
In the OAuth Endpoints field, select Custom.
Specify your OAuth 2.0 endpoint and the client ID you assigned to Google in the corresponding fields.
In the Step 1 section, don't select any Google scopes. Instead, leave this field blank or type a scope valid for your server (or an arbitrary string if you don't use OAuth scopes). When you're done, click Authorize APIs.
In the Step 2 and Step 3 sections, go through the OAuth 2.0 flow and verify that each step works as intended.

You can validate your implementation by using the Google Account Linking Demo tool.

In the tool, do the following steps:

Click the Sign-in with Google button.
Choose the account you'd like to link.
Enter the service ID.
Optionally enter one or more scopes that you will request access for.
Click Start Demo.
When prompted, confirm that you may consent and deny the linking request.
Confirm that you are redirected to your platform.